To answer this question, we first though we need to understand what the purpose of the tax code is. Surprisingly, the primary purpose is NOT to raise revenue. Revenue is incidental to the US taxing system.
This is the most eloquent, efficient description of the purpose of the US income tax:
[It is the] product of an unholy combine between statist intellectuals with visions of state-sponsored utopias, envious demagogues and the desire by established, wealthy interests to prevent any competition to their place and to offload business costs to an expanding regulatory welfare state.
If you disagree with the above statement, the way the Congress uses the IRS seems unpredictable and illogical.
If you agree, you can anticipate the IRS's next move.
Source "The Political Economy of the Origin and Development of the Federal Income Tax, by Bennett D. Baack and Edward John Ray, in Emergence of Modern Political Economy, ed. Robert Higgs (AI Press, 1985), pp. 127–31.